glue entries

What are glue entries and how do they work in OpenLDAP? The manageDSAit
code in back-bdb is intertwined with is_entry_glue() calls which I don't
know what to make of. The only glue I can see bdb create seems to be a
fake entry named "" if that is the database suffix. And I don't see
any mention of glue entries in other backends.
--
Hallvard